SHOCK T U B E STUDY O F T H E
3169
ACETYLENE-OXYGEN I~EACTION
have also been discussed for aromatic systems.21~22 Professor H. Bredereck of Technischen Hochschule These effects might bear some relation to the plots Stuttgart, Professor E. Hayashi of Shizuoka I’harriiain Fig. 1. However, I‘ig. 1 can, at any rate, provide a ceutical College, Drs. 1’. Kubota, S. Sumiriioto, R. useful relationship for predicting the magnitude of Konaka, and AI. Ogata of this laboratory for supplying us with several samples used. values in a molecule from the J C Hvalues J”(o,rho observed or calculated.
Acknowledgments. We are greatly indebted
to
(22) N. Jonathan, S. Gordon, and B. 1’. Dailey, J . Chem. P h y s . , 3 6 , 2443 (1962).
Shock Tube Study of the Acetylene-Oxygen Reaction’
by R. F. Stubbeman and W. C. Gardiner, Jr. Department of Chemistry, T h e University of T e z a s , A u s t i n , Texas
78719
(Received M a y 91, 1904)
The reaction of acetylene with oxygen was studied over the temperature range 15002500’K. using shock tube techniques. Observations were niade of ultraviolet and visible chemilurnincscence, and of the hydroxyl radical by absorption spectroscopy. The ratio of oxygen to acetylene varied from 0.34 to 7.3. Previous findings about the appearancc of the cheniiluminescerice were in general confirmed. Comparison of the teinporal behavior of OH concentration and visible chemiluminescence showed that OH appearance was substantially delayed. Consequences of the phase relationship betwcen OH and chemiluminescence are discussed with reference to current proposals for the rriechanisin of the branching chain reactions. Proposals are inade for the origin of the chcniiluininescence which are in agreement with conclusions from other studies.
Chemical kinetic investigations of the oxidation of acetylene have been undertaken using conventional methods for inany years. Spectroscopic investigations of acetylene flames, in particular, have proved fruitful in providing information about sonic of the possible reaction pathways. Renewed interest in this reaction has arisen with the continued development of thc lowpressure flame structure and shock tube techniques at high temperatures and the discharge-flow technique at low temperatures. The conibined resources of the several experimental approaches now being pursued promise that an understanding of the major elementary reactions involved, at least in the stoichiometric or lean flames a t low pressure, may be found in the near future. We report here a study of the acetylene-
oxygen reaction in shock waves in the temperature range 1500-25OO0Ied gas. Since the concentration of reactive mixture was always small coinpared to the concentration of argon, no substantial tenipcrature error would be involved if the vibrational equilibration was inconiplcte before onset of chcniical reaction. Temperatures quoted in this paper arc all no-reaction shock temperatures. A nionochroniator (Beckman DU-It) was used to isolate spectral regions of light transniitted through or emitted from the experiniental gas in the shock tube at the principal observation station. E’or cniission nieasurcnients, defining slits 1 111111. wide wero placed directly in front of the quartz windows, and the optical system was carefully aligned to bc norinal to the gas flow. For absorption measurenient~of 0 I I concentration, a bisniuth at)oniic line at 3067 A. was excited by a 24,50-4Ic. discharge in argon containing a tracc of bisniuth trichloridc.2 The light passed through defining slits on both sides of thc shock tube and a a s rcflcctcd by a concave niirror into the nionochroniator. The light intc~isityat the exit, slit of thc nionochroniator was measured by a 11’28 photoniultiplier. I:or 011 absorption iiieasurcnituts, the anode current of the 11’28 wa5 matched to cable inipcdance by n rathodc follow3r and recordcd on an oscilloscope. I’or so1iic mcasurcnmits of cniission, the 11’28 anode current was amplified and convc.rt6.d to logarithmic scale by a Kane log voltagc coiuprcssor before oscilloscopc 1-crording. T h c b over-all frcqucncy rcxsponse of thc detection systcnis could be increased to over l l l ~ . ; howcwlr, rapacitivc loading was ust’d to limit tlie frcThr .Journal
o,f
Physirnl Chtmislru
quvncy rcsponst’ to about 200 kc. i n niost experinients. Iight cwiission was nieasurcd a t the auxiliary observation station through glass-type and intcrfrrericc’ filters (in niost of- thr cxperinients Leported hrrc a Haird.itoniic Type 131, peak 4320 .k, band width at halfpeak traniniission .TO%)by nieans of anpeak 6.5 other 11’28 photoninltiplicr. It was ascertained by obscrvatioii of the saiiie quantity (4320 A. eniission) a t both stations that thc alignnient of the two stations corrtspondcd to r ~ ~ o l u t i oofn better than 1 psec. of laboratory t h e . A Tektronix Type 502 or 535.4, with CX or 11 preamplifier, was used together with oscillosc.opc. ca1ric1rasfor data recording. Experiiriontal mixtures w r e prepared in a conventional vacuuni systein. Coiiiposition was deterniincd by careful manonictric nieasiir~nients. Honiogcncity of cxpcriniental niixturcs was obtained by allowing sevcral days to elapse hrfore withdrawal of expcriiiicntal gas. Xrgon, oxygen, and acetylcnc were takvn from roinincrcial cylinders. Cursory procedures were used to reniove acetone from the acrtylenc, and the othw gascs were used with no purification. Llass spclctroiwtric analyses showed no inipurities. I’ress u m in the shock tube before each run wcr(’ nieasurcd with a Ihbrovin gauge. The starting prcssure in virtually all cxperinients reported here was 5 min. Tablc I lists the coinposition of the experiincntal gases. Xrgon was used as inert diluerit in each case.
K.,
Table I : Composition of Experimental ,\fixtures Mixture
no.
8
9 20 21 23 24
% C2H2
% Or
0 734 1 326
5 583 1 940
1 000
1 530
1 000 1 000 1 000
3 235 i 2iO 0 342
i 60 1 46 1 33 3 235 i 27 0 34
Results The cxpcriniental quantities reported here arc induction tinics for appearance of OH, induction times for visit)](>cniission near 2320 to rise to 0.1 of its niaximum peak height, p ~ a heights k for this visible radiation, and time constants for the exponential rise of this visible ~adiation. Seniiciuarititative n~oasurcnients wcw made of peak hcights for ul;raviolet radiation in t h e spectral rcgion 2000--2200 .I., arid induction tinics for this radiation w e r ~cstiniated by comparison with siniultrtncously rworded visible emission.
K.
(2) T.Carrington, .I. Chrm. Phvs.. 31, 1418 (1969)
X sample cxprrinwntal record of visihle emission and
OH ahsorption is shown in Iig. 1. The pulse form of the visihlc rmission is i n accord with thc rrsults of Hand and I F. Stuhhemnn and IT. C . Canliner. Jr.. .I. Chem. P h i i s . . 40. l i 7 1 (18Fd). T h e emission WR erroiieo~slyattrihuted to CO 0 in this romniunicntion. S i m t m l aiinlysis of the visible einksicm (G. 1'. Glass. private c o m ~ i ~ i i r i i c a t i ~smh)o w that CII i s llie iwiiic i i m l emitter at these WILW lewths. ( 5 ) (a) C . W. Hand. .I. C h m . I'hys.. 36, 2521 (1962); (h) (;. 13. Kirtinkowsky atid L. U . Hirhnnls. ihid.. 36, 1707 (191i2). ( 6 ) G. L. Srhott and J. L. Kinses. ihid.. 29. 1177 (185111.
+
3172
R. F. STUBBEMAN AND W. C. GARDINER, JR.
n
X
4.0
4.5
5.0
I/T
x lo4
5.5 (OK)-'
6.0
6.5
7.0
Figure 3. Induction times for blue emission to rise to 0.1 of the maximum. Solid line is the least-squares line of Kistiakowsky and Richards, determined by ultraviolet emission. Crosses are [O,] : [CZHz] = 7.3 :1. Circles are [OZ]: [CZHz] = 3.3:1. 45
50
55
iooo/r
60
65
70
OK-'
Figure 2. Induction times for appearance of OH. Solid line is the least-squares line of Kistiakowsky and Richards, determined by ultraviolet emission. Triangles are [OZ]: [CzHz]= 7.4:1. Circles are [O,] : [CZH2] = 3.2 : 1 . Crosses are [O,] : [CzH,] = 1.5 : 1. Gnits of ordinate are moles/l. X see.
dicative of the role of oxygen in the induction period reaction^.^ The solid line in the figure is the leastsquares line determined by Kistiakowsky and Richards from their observations of far-ultraviolet emission under experimental conditions similar to those employed in this work except for their use of starting pressures of 1 mm. compared to our starting pressure of 5 mm. It is clearly seen that the induction times for appearance of OH are substantially longer than those for onset of far-ultraviolet cheiiiiluiiiinescerice. The data points for the leanest mixture and those for the mixture stoichioinetric to CO and water appear to scatter uniformly together; those for the intermediate coniposition appear to correspond to shorter induction times, at least a t the lower temperatures. Defining an induction time for the onset of visible emission is a far more arbitrary affair than defining an induction tiine for appearance of OH. At the high sensitivities available when the logarithmic amplifier was used, scattered light began to register in some cases even before the shock wave arrived at the observation station, and the first part of the emission increase did not appear to be exponential. It was decided that The Journal of Physical Chemistru
a reasonable choice would be to take the time a t which the emission in a given shock rose to 0.1 of its peak height in that shock as the end of the induction zone for emission. The data are presented in Fig. 3. There is no apparent difference between the induction times for the two compositions used for these experiments. The solid line is once again the least-squares line of Kistiakowsky and Richards. The scatter of the points about this line is in accord with the previously mentioned observation that emission pulses in the visible and ultraviolet appeared to be coincident when observed in a single experiment a t the two observation stations. The peak heights of the visible emission are plotted against inverse temperature in Fig. 4. The ordinate is the output of the logarithmic amplifier, and hence a logarithmic measure of emission intensity, except at values less than about 0.02 v., where the amplifier response becomes linear rather than logarithmic. One decade corresponds to 0.11 v. The intensity is seen to be substantially higher for the 3 : 1 mixture than it is for the 7 : 1 mixture. Emission in the rich mixture is seen to be about three orders of magnitude weaker than in the 3 : 1 mixture. The peak intensity is strongly dependent on temperature; there is no suggestion, however, of a simple relationship. ~~~~
~~
~
(7) T. Asaba, W. C. Gardiner, Jr., and R. F. Stubbeman, paper presented t o Tenth Symposium (International) on Combustion, Cambridge, 1964.
SHOCKTUBESTUDYOF
THE
ACETYLEKE-OXYGEN REACTION
3173
IOOC
.3 0
..
ew
.
.2
VOLTS
o
o
oo
o
0
0 0
0 .
I
X X X 0 4
40
AA
45
,
1L 55
50
60 65 104 L'K)-~
4
70
75
x Figure 4. Peak heighta for blue emission. Ordinate is voltage output of logarithmic amplifier, and therefore a logarithmic measure of emission intensity except for the lowest values (see text). Circles are [OZ] : [ C Z H ~=] 3.2 :1. Crosses are [OZ]: [CZHZ] = 7.3: 1. Triangles are [OZ]: [CzHz] = 0.34: 1. I/T
Time constants for the exponentially rising parts of the visible emission pulses are shown as functions of inverse temperature in Fig. 5. The time constants for decadic rise obt,ained directly from experimental records were converted to gas time by multiplication by the shock density ratio, converted to the base e by division by 2.3, and adjusted to 1-mm. starting pressure by multiplying by 5, which assumes that the responsible processes are first-order. This allows a comparison with the data obtained for the same quantity by Hand.3 It is seen that there is a discrepancy of about a factor of three in the two sets of data.
5.5
4.0
4.5
5.0
x
TI
104
5.5
6.0
(OK)-'
Figure 5. Time constant to base e for exponential part of intensity rise of blue emission. The data are normalized to a starting pressure of 1 mm. Circles are data of Hand for two mixtures with [O,] : [CzHz] = 1.5 : 1. Triangles are our data for [O,] : [C2Hz]= 3.2: 1. Crosses are our data for [OZ]: [C2H,] = 7 . 3 : l . Starting pressures in Hand's experiments were near 1 mm., in our experiments near 5 mm.
covery of diacetylene in the mass spectrum of the shocked mixture by Bradley and Kistiakowsky. Subsequent experiments confirmed the presence of diacetylene in rich mixtures but indicated that CO was formed instead in lean mixturese; it was proposed that there was a reaction of C2H with oxygen competing with (4) C2H
+ 0 2 = CHO + CO
(5)
or
Discussion The work of Kistiekowsky and Richards, combined with the time-of-flight mass spectrometric study of Bradley and Kistiakowsky,* led to a proposal for the branching chain mechanism of the induction period having the form
+0 OH + CzHz = HzO -+ CzH 0 + C2Hz = OH + CzH CgH + CzHz = C4Hz + H H
+
0 2
=
OH
(1) (2) (3) (4)
The principal argunients advanced to support this chain were the analogy of the induction period behavior to that in the hydrogen-oxygen reactions and the dis-
C2H
+
0 2
=
2CO
+H
(6)
The chain-branching step 1 is here the same as in the hydrogen-oxygen reaction ; its presence seems wellconfirmed in light of the data of Kistiakowsky and Richards and the finding of Feniniore and Jones'o that oxygen is mostly consumed by H atoms in acetylene-oxygen flames. (8) J. N. Bradley and G. B. Kistiakowsky, J . Chem. Phys., 35, 264 (1961). (9) G. P. Glass, G. B. Kistiakowsky, J. V. Michael, and H. Niki, paper presented to the Tenth Symposium (International) on Combustion, Cambridge, 1964. (10) C. P. Fenimore and G. W. Jones, J . P h y s . Chem.. 63, 1834 (1959).
Volume 68, Number 11
ATovember, l Q S 4
3174
R. F. STUBBEMAK ASD W. C. GARDINER, JR.
A number of reactions have been proposed to account for the chemiluminescence and also for chemiionization1l,lZ in the acetylene-oxygen reaction.5a We consider first for the sequence leading to th: cherniluminescence in the far-ultraviolet and a t 4300 A.
+ CzH + 0 CH + 0 CH* + 0
CzH
0 2
+ CH* = CO + CH* = CO* + H = CO” + H =
COZ
(7) (8) (9) (10)
The starred CH refers to either the 2A or the 211 state, the starred CO to the A’II state. Abstraction of H from acetylene by 0 may compete with attack of the triple bond itself. Evidence from a flame study13 suggesting that the initial attack of 0 may be on the carbon atom has recently been strengthened by other flame in which the existence of a transient adduct CzH20was confirmed mass spectrometrically. At shock wave or flame temperatures this adduct would decompose rapidly ; two possible modes of decomposition would be
+ CH2 CzHZO(*?) = CH + CHO C2H20(3?)= CO
(11) (12)
It is likely that the double valence of the oxygen atom is preserved in its reaction with acetylene15; thus, if adduct formation is the principal fate of 0, then the decomposition products of the adduct should be reactive toward oxygen or acetylene molecules. The exothermic or slightly endothermic decomposition possibilities do not allow electronic excitation of the products, but they do open the possibility of the groundstate radicals CH, CHO, and CH2 participating in the main chain-branching reaction sequence. The requirement of atoniic oxygen, rather than OH or H, for producing CH chemiluiuinescence in acetylene has recently been demonstrated. Our results are in general accord with those proposals for the induction period reactions. Several additional features of the kinetics can be added by combining our results with those of previous workers. First let us consider the temporal behavior of the visible radiation and OH concentration illustrated in Fig. 1. The question of whether the onset of chemiluminescence or the appearance of OH corresponds to the beginning of macroscopic-scale chemical reaction is easily resolved; the onset of the radiation marks the end of the induction period. The suppression of OH appearance must then be explained. For reasons discussed p r e v i ~ u s l y this , ~ is probably a manifestation of the high reactivity of OH toward the reaction enT h e Journal of Physical Chemistry
vironment throughout the induction zone and the transition to the ((recombination region.”6 At first the only reaction partner is acetylene; when sufficient CO has been produced, the fast reaction
CO
+ OH
=
COz
+H
(13) will continue to keep the OH concentration below the limit of detectability. The final appearance of OH can be taken to be a measure of reactions such as (13) becoming reversible. This stage, under the conditions studied in our experiments, usually corresponded also to the virtual disappearance of chemiluminescence. The reasons for the rapid and virtually complete disappearance of the visible radiation are considerably more subtle than the reasons for the delayed appearance of OH. It is to be noted in Fig. 1 that the level of the emission falls through more than two orders of magnitude in a time comparable to the rise in concentration of OH. I n experiments a t higher temperatures, similar decays spanning at least four orders of magnitude can be seen. The familiar “radical overshoots” cannot begin to explain changes of this size a t the temperatures of these experiments. One is then drawn to the conclusion that a t least one of the reactants in the sequence leading to visible emission is a t an extremeZy low concentration in the equilibrium gas, and that one of the reaction partners populating the radiating state of CH is either acetylene itself or an intermediate formed from acetylene that does not appear in the equilibrium mixture in mole fraction greater than about 0.01 of the mole fraction of CO prevailing a t equilibrium. All of the oxygen-containing fragments in the main chain sequence above are present in substantial fractions of the HzO and CO concentrations except for CHO. Unoxidized carbon-containing fragments, however, are present only a t extremely low concentrations at equilibrium. Reactions such as (7) and @), therefore, are consistent with these requirements. It would be possible to make a decision as to which type of reaction is responsible for most of the emission if the exponential rise constant could be assigned to a reaction rate involving a product of two concentrations increasing exponentially in time, or a reaction rate involving only one exponentially increasing concentration. I n principle, this can (11) G. B. Kistiakowsky and J. V. Michael, J . Chem. P h y s . , 40, 1447 (1964). (12) G. P. Glass and G. B. Kistiakowsky, ibid., 40, 1448 (1964). (13) C. P. Fenimore and G. W. Jones, ibid., 39, 1514 (1963). (14) H. G. Wagner, private communication. (15) W. C. Gardiner, Jr., J . Chem. Phys., 40, 2410 (1964). (16) R. E. W.Jansson and K. D. Bayes, Proc. R o y . Soc. (London), t o be published.
SHOCK TUBESTUDYOF
THE
ACETYLENE-OXYGEN REACTION
be done by comparing the induction time for the process with the exponential rise time constant.B One can then estimate the number of decades through which the concentration or concentration product has risen a t the end of the induction period. Unfortunately, the present data do not give sufficiently reproducible results for this to allow a decision to be made, mainly because the temperature range of our experiments was too high for accurate comparative measurements of this kind. Careful work a t lower temperatures and concentrations may allow this distinction to be made. The above suggestions for the cause of the delayed appearance of OH do not lead immediately to any prediction about the dependence of the induction time for OH appearance upon composition, at least for the lean mixtures studied here. I n Fig. 2, the data appear to indicate that the induction periods for OH appearance are shorter for the 3 : 1 mixture than for either of the other two mixtures. The scatter of the data points is such that this effect cannot be held to be certain, but it is indeed indicated. Two different interpretations of such an effect would be possible, one as an effect involving the main branching chain, the other as an effect involving the transition to equilibrium which appears to be the precursor to OH appearance. Since the acetylene concentration was held constant in these experiments, it is necessary to propose competitive processes in order to predict a concentration of oxygen above and below whjlch the induction periods for O H appearance are lenglhened. Such processes are not indicated in any of the heretofore proposed kinetic steps, and no likely additional steps come to mind. Since the early work on composition dependence of the induction periods for ultraviolet emission led to a weak, monotonic dependence15bwe prefer to postpone any additions to the kinetic scheme that would allow interpretation of the apparent concentration dependence in Fig. 2 until the effect can be confirmed in inore accurate measurements over a wide composition range. The data for the inlduction time for blue emission to rise to 0.1 of its maximum are not useful for direct kinetic interpretation, but they do serve well to illustrate the coincidence of the appearance of visible and ultraviolet emission and to show the degree of agreement between ciur results and those of Kistiakowsky and Richards (Fig. 3). The scatter is in part due to the difficulty with scattered light, in our arrangement, and in part due to the normal variations encountered in mea,sureinents of this kind. We believe that there is no systematic error in our measurements of these data points other than that of the scattered light; it is our intention to test this in future experiments by investigating the phase relationship
3175
between ultraviolet and visible chemiluminescence with a detector more sensitive to the ultraviolet emission than the one used in the present experiments. The peak heights for the blue emission intensity are seen in Fig. 4 to increase very strongly on going from the 7:1 to 3:l mixture. At the higher temperatures, the difference is practically an order of magnitude in intensity. It does not appear that this is due in large measure to a more rapid reaction in the 3:1 mixture, since the data points for the induction period to 0.1 of maximum peak height scatter uniformly together. There is a possibility that the latter stages of the induction period are accelerated by a thermal effect, but this again does not appear likely in view of the moderate exothermicity of the proposed main chain, and the fact that the more likely effect here would be in the opposite direction than observed due to the increase in heat output which would accompany favoring reactions 5 and 6 over 4. The most likely explanation would appear to be a combination of effects strongly increasing the concentration of the reduced reaction partners in (7) and (8) (or, any of the similar reactions which can be written to populate CH*) when the ratio of oxygen to acetylene is reduced from 7 to 3. It is unfortunate that our present data do not include peak height measurenients extending to stoichioinetric or slightly rich compositions. The time constant for the exponential rise of the chemiluminescence intensity is intrinsically more satisfactory for comparison with predictions froni a kinetic scheme for the branching chain and population of the radiating states, but suffers in our experiments from lack of accuracy due to scattered light and lack of data for higher temperature ranges. It is particularly unsatisfactory that our data for the exponential time constant disagree with the results of Hand and Kistiakowsky (Fig. 5). The data of Hand and Kistiakowsky were taken for a mixture stoichiometric to CO and HzO (1.5:lratio of oxygen to acetylene) and were obtained in shock waves at starting pressures of 1 mm. Our correction for the different starting pressures was linear (multiplication by the ratio of starting pressures), and this may in fact be incorrect. If, as might seem likely, the over-all process leading to emission is approximately second-order, then a quadratic correction would be appropriate and the two sets of data would be in agreement. However, it does not seem likely that the over-all process leading to emission would be second-order in acetylene and zeroorder in oxygen. It can be seen in Fig. 5 that the 7:1 and 3 : 1 mixtures scatter together, indicating that there is no effect as large as first-order in oxygen. Perhaps a doubling of the time constant would be observed Volume 68, n'umber 11
Sonemher, 1904
3176
if the 3 : 1 mixture were compared with a 1.5: 1 mixture. Such a change would then confirm the suggestion that the two sets of data are actually in agreement and that second-order dependence upon pressure would be observed if stoichiometric mixtures were compared. More accurate values for exponential time constant(s?) for intensity rise of visible and ultra-
The Journal of Physical Chemistry
R. F. STUBBEMAN AND W. C. GARDINER, JR.
violet emission studied over wider ranges of temperature and composition should be quite valuable in assigning the mechanism of the population of the radiating states to specific elementary steps.
Acknowledgment. This work was supported by the U. S. Army Research Office (Durham).
